Mooring assembly

ABSTRACT

A mooring assembly for an offshore floater comprises a vertical shaft (4) in the floater (1), a rotating part (5) rotatably mounted in the vertical shaft about a vertical axis, a number of mooring cables (16) extending from the rotating part down to the seabed, and a device (17) aboard the floater (1) for tightening/slackening the mooring cables. Each mooring cable (15, 16) is secured, right below the rotating part (5), to a rigid body (13) having two ends, one end being connected by a universal swivel joint (12) to the rotating part (5) and the other end comprising a locking device (14) for locking the mooring line to the rigid body (13).

FIELD OF THE INVENTION

The invention relates to a mooring assembly for an offshore floater,said assembly comprising a vertical shaft in the floater, a rotatingpart rotatably mounted about a vertical axis in the vertical shaft, anumber of mooring cables extending from the rotating part down to theseabed, and means aboard the floater for tightening/slackening themooring cables.

BACKGROUND OF THE INVENTION

If a floater is moored offshore by means of a rotating part rotatablymounted in a vertical shaft, said rotating part will, when the floatermoves, be pulled along with it as a result of inertia and frictionalforces in the bearings of the rotating part in the shaft. The mooringcables, which are brought up into the rotating part, will be exposed towear and tear, and every endeavor is therefore made to use goodrotational bearing components which enable the rotating part to rotateeasily in relation to the floater.

The use of "inferior" bearing would be simpler and less expensive butwould give an undesired large cable divergence, with correspondinglygreater wear of the mooring cables.

SUMMARY OF THE INVENTION

An object of the invention is to make possible the use of simpler andless expensive bearings for said rotating part without thereby exposingthe mooring cables to greater hazard of wear.

According to the invention, a mooring assembly, as mentioned by way ofintroduction, is therefore proposed, said mooring assembly beingcharacterized in that each mooring cable right below the rotating partis secured to a rigid body having two ends, one end being connected by auniversal swivel joint to said rotating part and the other endcomprising a locking device for locking the mooring line to the rigidbody.

When the floater revolves about the rotating part under the influence ofwind and current, said rotating part will, as noted, have a tendency tobe pulled along during a part of the revolving movement, as a result ofinertia and frictional forces. Since the mooring line is brought intothe rotating part by means of the swivel-mounted rigid body, momentswill arise from the pointing movement, said moments resulting in therotating part being held back with greater force. This means that therotational bearings of the rotating part can be made simpler and lessexpensive since there is decreased dependency upon smoothly operatingbearings for the rotating part in the vertical shaft of the floater.

The angle of divergence between the rigid body and the mooring cablewill decrease corresponding to the increase in length of the rigid body.

The rotational bearings of the rotating part of the floater mayadvantageously comprise slide bearings.

The locking device for locking the mooring cable to the rigid body mayadvantageously be an activable chain stopper. This presupposes that themooring cable comprises a length of chain for engagement with the chainstopper in the rigid body.

It is particularly advantageous according to the invention that it ispossible, from each location on the rotating part to which a rigid bodyis connected, to form a vertical guide track, in the rotating part, foran elongated flexible element extending from a tightening/slackeningdevice mounted on the floater, such as a winch, saidtightening/slackening device being rotatable about an axis, at leastapproximately vertical, coinciding with the flexible element extendingtoward the device, for the removal of possible twists in the flexibleelongated element, which is connected to the mooring cable orconstitutes the mooring cable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall now be further explained by reference to thedrawings, wherein

FIG. 1 is a schematic cross section of a floater comprising a mooringassembly according to the invention,

FIG. 2 is a schematic plan view of the mooring assembly of FIG. 1, and

FIG. 3 shows a modified mooring assembly according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a floater which has a deck 2 and a bottom 3. Between thebottom 3 and the deck 2 there is a throughgoing vertical shaft 4 whereinthere is rotatably mounted a rotating part 5. The rotating part 5 ismounted for rotational movement about a vertical axis in the verticalthroughgoing shaft 4 by means of slide bearings 6, 7 and 8.

The vertical shaft 4 is at its upper end covered by a top 10. This top10 has a central aperture for letting through a central tube 9protruding from the rotating part 5.

In this case the rotating part 5 is provided with four vertical guidetracks 11 which are evenly distributed along its circumference. In FIG.1 only two such vertical tracks 11 are shown. At the bottom of eachtrack 11 there is mounted a universal joint 12, whereby an elongatedrigid body 13 is swivel-mounted in the rotating part 5.

The rigid body 13 is, as illustrated, at one end connected to the swiveljoint 12 and at its other end, protruding down into the sea, providedwith an activable chain stopper 14.

In connection with the rigid body 13, illustrated on the left-hand sideof FIG. 1, there is shown a mooring cable 15, extending from moorings onthe seabed. The mooring cable 15 is continued as a length of chain 16through the chain stopper 14 and the rigid body 13, and further up to awinch 17 on the deck 2 of the floater. The purpose of the length ofchain 16 is to make possible a locking of the mooring cable to the rigidbody by means of an activable chain stopper 14. Obviously, it is alsoconceivable that the mooring cable may continue as a regular cable upthrough the stopper 14 and the rigid body 13, and up to the winch 17,which in that case is a regular line winch, but the stopper 14 must thenbe correspondingly formed, i.e., as an activable stopper intended toretain a line or a cable fixedly.

After the mooring cable has been tightened sufficiently, it is locked tothe rigid body 13 and thereby to the rotating part 5 by means of thestopper 14. The connection with the winch 17 can then be slackened or,optionally, broken. The latter situation is shown at the right-hand sideof FIG. 1, where the chain 16 has been broken and is suspended from thestopper 14. Optionally, from such a broken length of chain a pilot linemay be extended to the rotating part and up through the track 11, sothat the length of chain 16 may be retrieved and connected to a winch ifa change in the tension of the mooring cable is desired.

It is quite possible to make use of different forms of mooring cableshere, both pure steel wires and chains, or combinations thereof, and soforth, since the essential matter is to obtain a locking of the mooringcable to the rigid body so that the latter will function as a connectingelement and a pointing element between the mooring cable and therotating part 5.

A mooring cable extending upward from the seabed to a floater will be ofconsiderable length, and it will often prove impossible to avoid theoccurrence of some twist and turn of the mooring cable. This isobviously undesirable. By mounting the winch 17 rotatably about anapproximately vertical axis coinciding with the extension of the cabletoward the winch, a twist or turn of the mooring cable can beeliminated. This can be achieved by the winch 17 being rotatablymounted, for example by means of a turntable, so that it can be rotatedwith a corresponding rotation of the connected mooring cable. In FIG. 1the position the winch 17 will occupy after a 180° rotation is shown bydotted lines; see also FIG. 2. By rotation of the winch, the undesirabletwist of the cable can be removed.

The whole of the winch, with a pertaining turntable or similarapparatus, can be moved round the whole circle, as indicated in FIG. 2,so that one and the same winch 17 thus can be used sequentially fortightening/slackening successive mooring cables 15, 16, brought upthrough the respective vertical guide tracks 11 in the rotating part 5.

FIG. 3 shows a possible embodiment according to the invention where themooring cables are attached to a buoy 20. In the rotating part 5' thereis formed a conical reception site 21 for the buoy 20. Here one usesprior art involving a buoy moored in a submerged state, capable of beingraised under the floater and connected thereto, i.e., to a rotating partof the floater. Optionally, the buoy may constitute a rotating part initself, since the buoy may be in two parts, one part being connected tothe mooring cables and rotatably mounted in an exterior part, which isfixedly connected to the floater.

What is claimed is:
 1. A mooring assembly for a floater at sea, saidassembly comprising a vertical shaft (4) in the floater (1), a rotatingpart (5) rotatably mounted about a vertical axis in the vertical shaft,a number of mooring cables (16) extending from the rotating part down tothe seabed, and means (17) aboard the floater (1) fortightening/slackening the mooring cables,wherein each mooring cable (15,16) right below the rotating part (5) is secured to a rigid body (13)having two ends, one end being connected by a universal swivel joint(12) to said rotating part (5) and the other end comprising a lockingdevice (14) for locking the mooring line to the rigid body (13).
 2. Amooring assembly according to claim 1,wherein the rotational bearings ofthe rotating part (5) in the floater (1, 4) comprise slide bearings (6,7, 8).
 3. A mooring assembly according to claim 1,wherein said lockingdevice (14) is an activable chain stopper.
 4. A mooring assemblyaccording to claim 1,wherein there, from each location (12) on therotating part (5) to which the rigid body (13) there is connected, isformed a vertical guidance track (11) in the rotating part (5) for anelongated flexible element (16) extending from a tightening/slackeningdevice (17) mounted on the floater (1), said tightening/slackeningdevice (17) being rotatable about an axis, at least approximatelyvertical, coinciding with the extension of the flexible element (16)toward the device, for removal of any possible twist in the flexibleelement, which is connected to the mooring cable (15) or constitutes themooring cable.
 5. The mooring assembly according to claim 4,wherein thetightening/slackening device (17) is movable along and within thecircumference of the vertical shaft (4).
 6. A mooring assembly accordingto claim 1,wherein the rigid body at said one end is connected by auniversal swivel joint to the rotating part via a buoy (20) moored bymeans of the mooring cables and capable of being brought into a lockingengagement with the rotating part.